1. Field of the Invention
The present invention relates to a drive method for a plasma display panel, and particularly to a drive method for an AC memory type plasma display panel.
2. Description of the Related Art
Plasma display panels (hereinbelow abbreviated "PDP") typically offer many features including thin construction, freedom from flicker, and a high display contrast ratio, and in addition are relatively easy to apply to large screens. They have a high response speed, and are the emissive type which can display color images using a phosphor. As a result, PDP are becoming increasingly widely used in recent years in the fields of computer-related display devices and color image display devices.
Depending on the method of operation, plasma display panels can be divided between the AC type, in which electrodes are covered by a dielectric and which are operated indirectly by AC discharge, and the DC type, in which the electrodes are exposed in a discharge space and which are operated directly by DC discharge. The AC type can be further divided between the memory type, which employs the memory of discharge cells as a drive method, and the refresh type, which doesn't make use of the discharge cell memory. The luminance of a PDP is proportional to the number of discharges, i.e., the number of repetitions of the pulse voltage. In the case of the above-described refresh type, luminance decreases with increase in the display capacity, and these displays are therefore mainly used in PDPs having a low display capacity.
FIG. 1 is a sectional view showing an example of the construction of one display cell in an AC memory type PDP. This display cell is made up of two insulator substrates 1 and 2 composed of glass, one being the front plate and the other being the rear plate; transparent scan electrode 3 and transparent sustain electrode 4 formed on insulator substrate 1; bus electrodes 5 and 6 arranged over scan electrode 3 and sustain electrode 4 to decrease the electrode resistance; data electrode 7 formed on insulator substrate 2 orthogonal to scan electrode 3 and sustain electrode 4; discharge space 8 between insulator substrates 1 and 2 filled with a discharge gas composed of, for example, helium, neon, or xenon, or a mixture of these gases; phosphor 11 for converting the ultraviolet rays generated by the above-described discharge of discharge gas into visible light 10; dielectric layer 12 that covers scan electrode 3 and sustain electrode 4; protective layer 13 composed of, for example, magnesium oxide for protecting dielectric layer 12 from discharge; and dielectric layer 14 that covers data electrode 7.
Explanation is next presented regarding the discharge operation of a selected display cell with reference to FIG. 1. When a pulse voltage that exceeds the discharge threshold value is applied between scan electrode 3 and data electrode 7 and discharge begins, positive and negative charges corresponding to the polarity of this pulse voltage are drawn to the surfaces of dielectric layers 12 and 14 on both sides, and charge is accumulated. The polarity of the equivalent internal voltage arising from this accumulation of charge, i.e., wall voltage, is the reverse of the polarity of the above-described pulse voltage, and the effective voltage inside the cell therefore decreases with the growth of discharge. Even if the above-described pulse voltage maintains a uniform value, discharge cannot be sustained and eventually stops. A sustaining discharge pulse, which is a pulse voltage of the same polarity as the wall voltage, is subsequently applied between neighboring scan electrode 3 and sustain electrode 4. This voltage combines with the wall voltage portion as the effective voltage, whereby the display cell exceeds the discharge threshold value and can discharge even if the voltage amplitude of the sustaining discharge pulse is low. Discharge can be sustained in the display cell by continually applying the sustaining discharge pulse alternately between scan electrode 3 and sustain electrode 4. This function is the memory function mentioned hereinabove. The above-described sustaining discharge can be stopped in the display cell by applying a wide low-voltage pulse or a narrow pulse, which has about the sustaining discharge pulse voltage, to scan electrode 3 or sustain electrode 4 so as to neutralize wall voltage.
FIG. 2 is a schematic plan view showing the composition of a PDP formed by arranging the display cell shown in FIG. 1 in a matrix. In the figure, PDP 15 is a dot matrix display panel in which display cells 16 are arranged in mxn rows and columns. PDP 15 is provided with scan electrodes Sc1, Sc2, . . . , Scm and sustain electrodes Su1, Su2, . . . , Sum arranged in parallel as row electrodes. PDP 15 is also provided with data electrodes D1, D2, . . . , Dn arranged as column electrodes orthogonal to the scan electrodes and sustain electrodes.
FIG. 3 is a waveform chart of drive pulses illustrating a conventional drive method (hereinbelow referred to as the "first example of the prior art") for the PDP shown in FIG. 1 proposed in the "International Symposium Digest of Technical Papers of the Society for Information Display," Volume XXVI, October 1995, pp. 807-810.
In FIG. 3, Wc is a sustain electrode drive pulse applied in common to sustain electrodes Su1, Su2, . . . , Sum; Ws1, Ws2, . . . , Wsm are scan electrode drive pulses applied to scan electrodes Sc1, Sc2, . . . , Scm, respectively; and Wd is a data electrode drive pulse applied to data electrode Di (1.ltoreq.i.ltoreq.n). One drive period (one frame) is made up of priming discharge interval A, addressing discharge interval B, and sustaining discharge interval C, and image display is obtained through repetition of these intervals.
Priming discharge interval A is the interval for generating wall charge and active particles in the discharge space so as to obtain stable addressing discharge characteristics in addressing discharge interval B. In priming discharge interval A, after applying priming discharge pulse Pp to cause all display cells of PDP 15 to discharge simultaneously, a priming discharge erasing pulse Ppe is simultaneously applied to each scan electrode to eliminate any charge of the wall charge generated by the priming discharge interval that would impede addressing discharge and sustaining discharge. In other words, priming discharge pulse Pp is first applied to Su1, Su2, . . . , Sum, and after discharge occurs in all display cells, erasing pulse Ppe is applied to scan electrodes Sc1, Sc2, . . . , Scm to bring about erasing discharge and eliminate wall charge accumulated due to the priming discharge pulse.
In addressing discharge interval B, sequential scan pulse Pw is applied to each scan electrode Sc1, Sc2, . . . , Scm. Synchronized with this scan pulse Pw, data pulse Pd is selectively applied to data electrodes Di (1.ltoreq.i.ltoreq.n) of those display cells that are to display, thereby bringing about addressing discharge and generating wall charge in display cells that are to display. Scan base pulse Pbw is a drive pulse that is applied to all scan electrodes in common throughout the duration of the addressing discharge interval, and is set to an amplitude whereby discharge does not occur between a scan electrode and data electrode even if data pulse Pd is applied to a data electrode.
In sustaining discharge interval C, sustaining discharge pulse Pc of negative polarity is applied to sustain electrodes, and sustaining discharge pulse Ps of negative polarity having phase delayed 180.degree. from that of sustain electrode pulse Pc is applied to each scan electrode, thereby maintaining the sustaining discharge necessary for obtaining the desired luminance in display cells in which addressing discharge has occurred in addressing discharge interval B.
FIG. 4 is a waveform chart showing the drive method of the prior art described in Japanese Patent Laid-open No. 68946/97 (hereinbelow referred to as the "second example of the prior art").
In FIG. 4, Wc is a sustain electrode drive pulse that is applied in common to sustain electrodes Su1, Su2, . . . , Sum (corresponding to an X sustain electrodes in Japanese Patent Laid-open No. 68946/97); Ws is a scan electrode drive pulse applied to each of scan electrodes Sc1, Sc2, . . . , Scm (corresponding to Y scan electrodes in Japanese Patent Laid-open No. 68946/97); and Wd is a data electrode drive pulse applied to data electrodes Di (1.ltoreq.i.ltoreq.n) (corresponding to address electrodes in Japanese Patent Laid-open No. 68946/97). One period of drive (one frame) is made up of priming discharge interval A (corresponding to a reset period in Japanese Patent Laid-open No. 68946/97), addressing discharge interval B (corresponding to an addressing discharge interval in Japanese Patent Laid-open No. 68946/97), and sustaining discharge C, and the desired image display is obtained through the repetition of these intervals.
The principles of driving priming discharge interval A, addressing discharge interval B, and sustaining discharge interval C are as in the first example of the prior art, and explanation is therefore here omitted. In sustaining discharge interval C, the leading sustaining discharge pulse is a negative-polarity sustaining discharge pulse (applied voltage -1/2 Vs; hereinbelow referred to as "negative-polarity 1/2 sustaining discharge pulse") substantially 1/2 the voltage of sustaining discharge voltage Vs that is applied to sustain electrodes, and a positive-polarity sustaining discharge pulse (applied voltage +1/2 Vs; hereinbelow referred to as "positive-polarity 1/2 sustaining discharge pulse") substantially 1/2 the voltage of sustaining discharge voltage Vs that is applied to scan electrodes. The second sustaining discharge pulses are a positive-polarity 1/2 sustaining discharge pulse that is applied to sustain electrodes and a negative-polarity 1/2 sustaining discharge pulse that is applied to the scan electrodes. These operations are repeated sequentially to maintain sustaining discharge (hereinbelow referred to as "bipolar sustaining discharge").
The difference between the first and second examples of the prior art lies in the sustaining discharge pulse: in the first example of the prior art, it is a negative-polarity sustaining discharge, and in the second example of the prior art, it is a bipolar sustaining discharge. The negative-polarity sustaining discharge does not make the phosphor surface on the data electrode side a cathode and can therefore prevent deterioration due to the sputtering of positive ions and thus promote longer life. In the bipolar sustaining discharge, on the other hand, voltage applied between the data electrode and scan electrode and between the data electrode and sustain electrode reaches a maximum of 1/2 Vs. As a result, discharge between the data electrode and the scan electrode or sustain electrode is suppressed, and sufficient sustain voltage is applied between the scan electrode and sustain electrode.
The use of a negative-polarity sustain pulse in the first example of the prior art, however, means that sustain voltage Vs is also applied between the data electrode and the scan electrode or sustain electrode, i.e., between opposite electrodes. As a result, the amplitude of sustain pulses must be lower than the initial discharge voltage between opposite electrodes, resulting in cases of insufficient sustain pulse voltage applied between the scan electrodes and sustain electrodes.
In the second example of the prior art, on the other hand, selected cells that undergo addressing discharge in the addressing period exhibit an unstable transition to discharge by the leading bipolar sustaining discharge pulse. The source of this instability can be explained as follows: First, a positive wall charge forms on the scan electrode and a negative wall charge forms on the data electrode and sustain electrode following addressing discharge in selected display cells. In the leading sustaining discharge pulse, discharge is generated by the positive-polarity 1/2 sustaining discharge pulse applied to the scan electrode and the negative-polarity 1/2 sustaining discharge pulse applied to the sustain electrode. At substantially the same time, the superimposed voltage between the positive-polarity 1/2 sustaining discharge pulse applied to the scan electrode and the positive wall charge on the scan electrode and the negative wall charge on the data electrode occurs discharge between opposite electrodes, thereby resulting in instability of sustaining discharge between the scan electrode and sustain electrode.
In particular, wall charge or active particles in the discharge space gradually diminish in display cells in which the time interval from addressing discharge to sustaining discharge is long. In such cells, the amount of charge is low at the time of applying a leading sustain pulse, and the leading sustaining discharge is therefore weak. If discharge between opposite electrodes occurs at the same time, the amount of charge that contributes to surface discharge is even further diminished, sufficient wall charge does not form on the scan electrode and sustain electrode in the leading sustaining discharge, and second and subsequent sustaining discharges are difficult to continue.